Method for determining direction-dependent properties of enamels

ABSTRACT

A method of determining direction-dependent properties of coatings, in which measurements of coating properties are made along a test track ( 4   a   , 4   b ) on a sample coating using one or more measuring instruments and at least one measurement is recorded in relation to direction ( 6 ). The course of the sample coating is such that there are points of identical coat thickness ( 2 ) with different coat-thickness gradients ( 5 ) along the test track.

The present invention relates to a method of determiningdirection-dependent properties of coatings, in which measurements ofcoating properties are made along a test track on a sample coating usingone or more measuring instruments.

For the development and quality control of paints and other coatingmaterials it is necessary to investigate the resulting coatings inrespect of a variety of properties. To this end, sample coatings areproduced for which the designation “test panels” has become established,on account of the fact that the sample-coated article is generallyplatelike.

The properties investigated with the aid of test panels relate to abroad spectrum of relevant properties of the coating. They include onthe one hand the optical properties, especially the color properties ofthe coating (shade, gloss, evenness, effect properties, haze, hidingpower). On the other hand, the mechanical properties are of interest,such as the hardness of the coating, its adhesion to the substrate, andelasticity. Finally of interest are further physical properties, such asthe diffusion capacity of foreign substances in the coat, the electricalconductivity of the coat, the UV absorbency, the lame retarding effect,and the resistance of the coat under stresses as encountered inpractice.

A variety of methods have been developed for the efficient measurementof the test panels. For instance, DE-196 40 376.6 describes an automatedmethod of measuring coated test panels. In that method a robot guidesvarious measuring instruments along predetermined test tracks over thetest panel, and electronically records the measurements obtained. Thedetermination of the coat-thickness dependency of various parameters ina single measuring operation is the aim of DE 196 05 520 C1. For thispurpose, a wedge-shaped coating film is applied and both the coatthickness and the optical parameters are measured in each case along agridlike screen.

However, the methods referred to have the disadvantage that they do nottake sufficient account of the directional dependency of themeasurements. In the case of effect coatings, for example, as widelyused in automotive finishing, however, the angular dependency of opticalproperties plays an important part. For the measurements to bemeaningful it is therefore vital to take into account the angularconditions relative to the coat surface under which said measurementswere taken, and to obtain sufficient measurements to allow recognitionof functional correlations.

In the case where the coat thickness varies, moreover, it may beimportant to know the situation of the measurement direction relative tothe coat-thickness gradient. This mutual dependency of measurementdirection and coat-thickness gradient is not taken into account, andcertainly not efficiently recorded, in any of the prior art methods.

In contrast, the present invention has set itself the object of avoidingthe disadvantages of the prior art and of providing a method which canbe carried out efficiently, simply, and automatically as well ifdesired, and which in one measurement pass makes it possible to detectdirection-dependent measurements and also a dependency of themeasurements on the coat-thickness gradient.

This object is achieved by means of a method in which measurements ofcoating properties are made along a test track on a sample coating usingone or more measuring instruments. At least one measurement is to berecorded in relation to direction, i.e., it depends on the relativeangle between the measurement direction and a second direction, e.g.,the film surface and/or the coat-thickness gradient. This coat-thicknessgradient is a two-dimensional parameter (vector) which points in thedirection of the steepest increase in coat thickness.

Moreover, the course of the sample coating and of the test track is suchthat there is at least one coat thickness of the sample coating whichoccurs at least twice and with different coat-thickness gradients alongthe test track. Once during the measurement along the test track, inother words, a coat thickness SD₀ is traversed where there is a certaincoat-thickness gradient G ₁ (increase or decrease in coat thickness),and this coat thickness SD₀ is subsequently traversed a second time witha different coat-thickness gradient G ₂.

The method of the invention has the advantage that, in a singlemeasurement pass (measurement along the test track), measurements aremade at different angles between measurement direction andcoat-thickness gradients G for at least one coat thickness SD₀. Anymutual dependency of these directions that leads to measurabledifferences is immediately recognized. This is important, for example,for many optical properties of effect coatings, in the case of whichsuch deviations are not desired.

Preferably, the corresponding coat-thickness gradients are different insign, i.e., they point to different sides of space and are of equalmagnitude (G ₁=−G ₂) Therefore, just the directional dependency of theparameter of interest on the coat-thickness gradient is detected, withother conditions remaining constant.

In the simplest case, the coat thickness along the test track will havea minimum or a maximum, i.e., its course will have the form of a troughor peak. Since there is a constant change in the coat thickness, aroundthe minimum/maximum, a continuous test track will traverse all coatthicknesses twice and with different gradients.

In particular, the coat thickness may change symmetrically along thetest track, i.e., plotted as a function of the location, the coatthickness produces a mirror-symmetrical line. Specific symmetricalcourses of this kind are, for example, bell-shaped or parabolic.

A sample coating of the aforementioned kind, with symmetry and athickness maximum, may be produced, for example, by spraying along astraight line. As a result of the normal distribution of the spray mistwith decreasing film thickness at the edges of the application, there isin fact automatically formation of a coat-thickness profile whichextends in a bell shape transversely to the spray direction.Consequently, sample coatings of this kind can be produced usingconventional methods and automatic equipment.

The test track may have a very general course. The expression “testtrack” refers quite generally to the temporally ordered sequence of themeasurement sites. The test track corresponds to the path traveled bythe measuring instruments over the sample coating, although only thosesites at which measurements take place are ultimately relevant. Forreasons of simplicity and mechanical operability of the measuringinstruments, the test track will generally extend without reversals, andin the simplest case will be linear.

Using the method of the invention it is possible to measure, inter alia,coat thickness, evenness, shade, haze, and/or gloss of the samplecoating. For all measurements, it is particularly preferred to recordthe coat thickness as well, in order to determine the dependency ofthese measurements on the coat thickness. Furthermore, it is possible inthat case to monitor the presence of comparable coat thicknesses and tomonitor the coat-thickness gradients in relation to the measurements.If, however, the coat-thickness course of the sample coating issufficiently constant and reproducible, it may be possible to refrainfrom such subsequent measurement and to derive or estimate the coatthickness indirectly from the site of the measurement.

In the text below, the invention is illustrated by way of example withreference to the figures.

FIG. 1 shows a perspective view of the measurement of the invention.

FIG. 2 shows the coat thickness as a function of the measurement path.

FIG. 3 shows measured brightness values as a function of the coatthickness.

FIG. 1 shows, in a perspective view, the principle of the measurement ofthe invention on a sample coating 1. The sample coating consists of asubstrate 3, e.g., a metal panel, and of a coating film 2 appliedthereon. The coat thickness of the coating film 2 shown has a curving,symmetrical course with c maximum in the center. The substrate 3 willgenerally be planar, as shown, although in principle it could also havean arbitrarily curved surface. In the case of a planar substrate,however, the conditions are more simple, since the film surfacecorresponds directly to the course of the coat thickness and thereforethe inclination of this surface and the coat-thickness gradient areparameters which correspond to one another.

If the substrate base were to change in a nonplanar manner, then thecoat thickness and its gradient would not be represented solely by thefilm surface. A limiting case in this context would be a planar filmsurface over a nonplanar substrate. In that case there would be nosurface inclination and hence only the isolated influence of thecoat-thickness gradient.

Also shown are test tracks 4 a and 4 b, of which one—4 a—extends in thex-direction, the other—4 b—in the y-direction. Drawn in on the tracksare exemplary gradient vectors 5 a, 5 a′ and 5 b, 5 b′, which should allbe situated at the same coat thicknesses SD₀. The gradients lie parallelto the plane of the substrate 3 and point in the direction of maximumincrease in coat thickness. If the measurement is performed along a testtrack 4 a, then measurement takes place first at the coat thickness SD₀with a positive gradient 5 a and subsequently at the same coat thicknessSD₀ with a negative gradient 5 a′.

Similar comments apply to the test track 4 b, which is traversed inparallel sections. In addition to the tracks 4 a, 4 b depicted by way ofexample, numerous other kinds of test tracks are also possible.

The vectorial gradients G are defined mathematically by way of thederivation of the function f(r), which in a coordinate system with(two-dimensional) site vector r describes the surface of the coatingfilm 2, i.e.:

  G :=grad(f)=∇f=df/dr

The arrow 6 symbolizes the viewing direction of a measuring instrument,e.g., of a calorimeter. While it is possible in principle for theviewing direction to change along the test track, it is an advantage ofthe method of the invention that it may remain constant (i.e., isshifted only in parallel). This considerably simplifies the guidance ofthe measuring instruments. With this setup, the directional variationsrequired for the measurement are obtained by means of the specificcourse of the coat thickness and of the test track, in accordance withthe invention.

FIG. 2 shows, diagrammatically, the plot of the coat thickness SD(x) ofthe coating film 2 from FIG. 1 over the path section x traveled on thetest track 4 a. The curved course can be seen, with the gradients G ₁and G ₂ at the coat thickness SD₀. Owing to the linear course of thetest track 4 a transverse to the curve of the coating, the gradients inthis case correspond to the derivation dSD/dx.

FIG. 3, finally, shows a real measurement of the brightness L* (CIELABsystem) in accordance with the principle illustrated in FIG. 1 (track 4a). In the diagram, the brightness is plotted as a function of the coatthickness SD. The corresponding coat thicknesses may be measured eitherat: the same time as the brightness or else calculated from a knowncorrelation between coat thickness SD and path section x traveled on thetest track (cf. FIG. 2).

The measurements plotted are shown with different symbols for the risingand the falling sections of the test track. Finally, for each coatthickness SD₀ between about 10 and 25 μm, there are two measurementswhich can be seen in relation to two different gradients. From the plotit emerges that these measurements diverge in the manner of ahysteresis, i.e., the coating looks different from two viewingdirections rotated by 180° (and under otherwise identical conditions). Adifference of this kind, however, is not tolerable, something which canbe discovered for the coating material in question directly on the basisof a measurement in accordance with the invention.

What is claimed is:
 1. A method of determining direction-dependentproperties of coatings comprising measuring at least one coatingproperties along a test track on a sample coating using at least onemeasuring instruments to give at least one measurement, wherein a) atleast one of the at least one measurement is recorded in relation todirection, b) the sample coating has at least one coat thickness thatoccurs at two points and at at least two different coat-thicknessgradients along the test track, and c) the measuring is done at least atthese two points.
 2. The method of claim 1, wherein the at least twocoat-thickness gradients are different in sign.
 3. The method of claim1, wherein the at least one coat thickness has a minimum or a maximumalong the test track.
 4. The method of claim 1, wherein the coatthickness changes symmetrically along the test track.
 5. The method ofclaim 4, wherein the coat thickness changes symmetrically along ie testtrack in a bell-shape.
 6. The method of claim 4, wherein the coatthickness changes symmetrically along the test track in a parabolicshape.
 7. The method of clam 1, which is used to measure coat thickness,evenness, shade, haze, and/or gloss of the sample coating.
 8. The methodof claim 1, wherein the sample coating is produced by sprang along astraight line.
 9. The method of claim 1, wherein the test track extendswithout reversals.
 10. The method of claim 9, wherein the test trackextends linearly.
 11. The method of claim 1, wherein the measuring isdone in one pass along the test track.
 12. The method of claim 1,wherein the measurement recorded in relation to direction is based on arelative angle between a measuring direction and a second direction. 13.The method of claim 12, wherein the second direction is related to thesurface of the sample coating.
 14. The method of claim 12, wherein thesecond direction is relative to the coat thickness gradient.